Flat-rolled steel products fall within the categories of hot-rolled and cold-rolled bands, either constituted as sheets having a width over 300 mm and a thickness of at least 5.8 mm or strips having smaller dimensions. Typical cold-rolled material has a fine surface finish and can be reduced to a very small thickness, whereas hot-rolled material can be produced at substantially lower cost than the cold-rolled material.
With the hot-rolling process a room-temperature coil of a band having a thickness between 6.0 mm and 2.0 mm is first unwound. The band is then pickled or otherwise descaled and heated up to the appropriate recrystallization temperature, above 700.degree. C. It is then run through a plurality of stands of finishing rollers to reduce its thickness. Thereafter the band is again cooled below the recrystallization temperature and passed through finishing rollers which reduce its thickness slightly and give it a good surface finish. The reheating and recooling of the material requires considerable energy, while at the same time the system uses a considerable amount of equipment.
It has been attempted to reduce the thickness of a relatively wide sheet having a width over 600 mm to a thickness of between 0.5 mm and 2.0 mm, normally between 0.7 mm and 1.5 mm, using hot-rolling exclusively. It has been found extremely difficult to maintain the proper temperature in such a sheet so as to produce a band of even width, uniform section, and good planarity. Thus it is normally considered impossible in a hot-rolling operation to reduce the band thickness below 2.0 mm or 1.5 mm at the most. Many attempts to reduce a band by hot-rolling alone to a desired relatively small thickness have proven unsuccessful, even with the use of complex electronic thickness-monitoring and control systems. Frequently the band will curve to one side, so that when subjected to subsequent rolling a fold will be formed. Thus recourse must invariably be had to a combined hot-rolling and cold-rolling system with the hot-rolling reducing the band thickness considerably, then the cold-rolling producing the desirable smooth surface finish and dimensional regularity.
Another disadvantage of the hot-rolling method is that the surface of the band being worked is frequently embossed by the hot-working rollers. Considerable subsequent cold-rolling is necessary to finish such a product and remove the irregularities on its surfaces. Even when this is done the planar anisotropy of the product is often problematic. Thus subsequent working of such sheets, as for instance in stamping or cutting, becomes very difficult in one direction in the plane of the sheet. This is a particular problem when silicon steel sheets are being worked for subsequent use in transformers, motors, or the like. The magnetic anisotropy can greatly reduce the efficiency with which such a plate transmits magnetic forces or currents, so that extreme care must be taken in rolling out sheets for such use. The particular surface formation on the hot-working rollers determine in part the extent of this anisotropy. The production of the desired roller texture is also relatively expensive.